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Evolution in Structured Populations

Group structure, Ecology and Heritability.

One of the interesting points raised by the community selection experiments I talked about last week is the issue of the mode of founding of new units. When we think about genes and individual level traits in a typical sexual organisms this is a non-issue. Every individual has two parents, and each contributes half of the patterning compartment (genes) to the offspring. When we start moving to different levels of organization and different modes of inheritance the pattern is no longer invariant. This raises the important point that what is heritable, and what types of traits selection can act on is not only a function of the level at which selection acts, but also the ecology and the process of the founding of new units.

This is not a new idea. In the recent history of multilevel selection the first discussion of this that I am aware of is in Wade’s quarterly review of biology (1978. Qrt. Rev. Biol. 53: 101-114.). In this review he makes the important distinction between a “migrant pool” and a “propagule pool” form of group founding.

The important point here is that in the migrant pool individuals move independently to found the next generation. As a result interactions among individuals are broken up at each founding event, and genetic differences are homogenized. In contrast, in the propagule pool, groups of individuals move together to found the next generation of groups. This means that social interactions and mating structure are preserved, and genetic differences among groups are maintained rather than being homogenized.

There are actually two processes going on here. First, there is the genetic homogenization. This is primarily concerned with what might be termed the field of recombination. I have already discussed how in a large panmictic population epistasis can be ignored, but in small populations gene interactions can be statistically converted to additive genetic variance. Population structure, by changing gene frequencies within the group, and excluding some alleles at interacting loci, has the effect of driving this conversion, and in effect giving “heritability” to these epistatic interactions. From a genes eye view this would equate to limiting the field of interacting partners, giving more reliability to the genotypic effect our focal allele, and assuring that sexual reproduction does not completely scramble the epistatic interactions. It may seem odd to think of population structure as an ecological means of modifying linkage and recombination, but from a genetic standpoint that is exactly what it does. Returning to the migrant pool versus the propagule pool, it should be apparent that the propagule pool will do a much better job of preserving these genetic combinations and limiting the field of recombination than will the migrant pool.

The second important point is the interactions among individuals. That is, with the migrant pool the individuals that are interacting will have parents drawn randomly from the metapopulation, and interacting partners will not have predictable behaviors and physiologies. In contrast with the migrant pool, these ecological interactions will be preserved, since the parents of interacting individuals would also have interacted. The migrant pool makes these ecological interactions predictable and thus heritable.

This can be made much clearer if we think about community selection. For community selection we can keep the migrant pool model, but we can add nuance to the propagule pool model. In particular we can distinguish between single species and the multi-species propagule pools (note I have changed the color scheme. A is species A, B is species B, and I have dropped the subscripts to simplify the drawing) (Goodnight 2011. Phil. Trans. R. Soc. B 366: 1401-1409).

In the community selection experiment I described last week I used a multispecies propagule pool. This means that every generation the same strain of the two species were transferred together. As a result the interactions between the two species became heritable. Thus, a change in the predation rate of T. confusum that affected the population size of T. castaneum would be heritableat the community level and could contribute to the response to community selection on T. castaneum population size.

In that experiment I saw clear evidence that interactions between the species was contributing to the response to selection. It is tempting to speculate that had I used a single species propagule pool model to found the next generation I would not have seen these interspecies interactions contributing to the response to selection (I tried to get NSF funding for that once, but well, we all know how such things go, and now I am too allergic to the beetles to do the experiment).

The other thing that I have not talked about, mainly because I don’t have any clear ideas about it, is the effect of propagule size on group heritability. It seems reasonable to speculate that on the one hand smaller propagules should increase the rate of divergence of populations, but on the other hand, lower the heritability of group characteristics. I have no idea where these two forces would balance out, although some experimental data (e.g., Swenson, Wilson and Elias 2000. P.N.A.S. 97: 9110-9114; Wade 1982. Evolution 36: 945-961) suggest that the answer will not necessarily be simple.

What about nature? Well, it is pretty unimaginable that whole forest biomes migrate together to found a new community on bare ground. We all love Clements, Shelford, and Emerson, but I, and I dare say all, or at least nearly all, modern biologists have to remain skeptical about their expansive view of the “superorganism”. However, it is quite reasonable that parts of a community may migrate together to found new communities. For example, more than half of the cells in a typical human belong to some other species. When we migrate to a new location we bring our gut fauna with us. At a macro level, many organisms carry ectoparasites (fleas and lice) and phoretic organisms (seeds and some mites) with them as they move from place to place. Thus, at this level multispecies propagule pools do not seem that unreasonable. I suspect the reality is that real communities are founded by a mixture of individual migrants, single species propagules, and multispecies propagules. How much these different modes contribute to the heritability of traits is an interesting question, and one that I suspect can only be answered empirically.

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